Do not restore immediately when GeckoSession.ContentDelegate.onKill() is getting called

[pocmo]

When the content process gets killed by Android (to reclaim resources) then we immediately create a new GeckoSession and restore the last state we have:
https://github.com/mozilla-mobile/android-components/blob/master/components/browser/engine-gecko-nightly/src/main/java/mozilla/components/browser/engine/gecko/GeckoEngineSession.kt#L648-L670

Since this should behappening while the app is in the background and the system is low on memory, restoring immediately can cause issues and loops of "kill and restore".

For content process crashes we do something else. We also create a new GeckoSession but do not restore. Instead we mark the session as crashed and notify the app. The app then usually shows some "This tab crashed. Do you want to restore?" UI and then the user can decide to restore or close the tab.
https://github.com/mozilla-mobile/android-components/blob/master/components/browser/engine-gecko-nightly/src/main/java/mozilla/components/browser/engine/gecko/GeckoEngineSession.kt#L640-L646

For content process kills we should delay restoring (and I guess recreating the GeckoSession?) until the tab is the next time rendered (when the app gets resumed).

┆Issue is synchronized with this Jira Task

[pocmo]

This would be a thousand times easier if EngineSession instances wouldn't exist in both, SessionManager and BrowserStore. I will take a little detour and see if we are ready to move them completely.

[pocmo]

Yep, we will do the move first:
#7867

[pocmo]

This was fixed in #8310 / #8255.